DEVELOPMENT OF VERTEBRATES. 131 



quently owing to the greater convenience of early development, 

 the two systems might acquire a development from the same 

 mass of cells and those the cells of the inner or hypoblast layer, 

 so that the derivation of the body muscles from the hypoblast 

 would only be apparent and not real, or (2) owing to their 

 being better nourished as they would necessarily be, and to 

 their possibly easier adaptability to some new form of move- 

 ment of the animal, the muscle-cells of the alimentary canal 

 might become developed exclusively whilst the original mus- 

 cular system atrophied. 



I only hold this view provisionally till some better explana- 

 tion is given of the cases of Sagitta and the Echinoderms, as 

 well as of the nearly universal derivation of the mesoblast from 

 the hypoblast. The cases of this kind may be due to some 

 merely embryonic changes and have no meaning in reference 

 to the adult condition, but I think that we have no right to 

 assume this till some explanation of the embryonic can be 

 suggested. 



For vertebrates, I have shewn that in Selachians the body 

 cavity at first extends quite to the top of what becomes the 

 muscle plate, so that the line or space separating the two layers 

 of the muscle plate (vide Balfour, ' Development of Elasmo- 

 branch Fishes 1 ,' Quart. Jonrn. of Micro. Science for Oct., 1874. 

 Plate XV, fig. n a, 11 /;, 12*7, ;/>.) is a portion of the original 

 body cavity. If this is a primitive condition, which is by no 

 means certain, we have a condition which we might expect, 

 in which both the inner and the outer wall of the primitive 

 body cavity assists in forming the muscular system of the 

 body. 



It is very possible that the formation of the mesoblast as two 

 masses, one on each side of the middle line as occurs 'in Sela- 

 chians, and which as I pointed out in the paper quoted above 

 also takes place in some worms, is a remnant of the primitive 

 formation of the body cavity as paired outgrowth of the ali- 

 mentary canal. This would also explain the fact that in Sela- 

 chians the body cavity consists at first of two separate portions, 

 one on each side of the alimentary canal, which only subse- 



1 Paper No. V, p. 60 et set/, of this edition, pi. 4, figs. 1 1 a, i i h, 12 a, nip. 



